Alternating-flow heat exchangers are heat exchangers in which gas or fluid may flow in at least two flow paths through the heat exchanger. Such heat exchangers have a variety of uses, such as, for example, as catalytic converters, furnace heat recuperators, turbine engine recuperators, and in process fluid heating or cooling. The same structure may also be used for cross-flow filters or bleed-through reactors.
Ceramic heat exchangers are typically formed by first extruding a honeycomb like body of ceramic material from a die orifice. The extrusion results in a block of ceramic material having flow channels or cells which are typically of square or other rectangular cross-section, arranged parallel and adjacent to one another along the axis of extrusion. Typically, to form alternating-flow heat exchangers, portions of the sides of the extruded block are commonly cut away to convert the original ceramic block, which originally had only straight-through passages, into one alternating between rows of straight-through flow (typically referred to as the primary flow), and rows of Z-flow, L-flow, U-flow or other similar cross directional flow (which are typically referred to as the secondary flow) through the ceramic block.
Such secondary-flow (Z-flow, L-flow, etc.) channels are commonly made by sawing into the sides of some of the channels in the ceramic block and afterwards sealing the ends of these channels, thereby forming the secondary-flow channels. Examples of methods which utilize sawing techniques to form Z-flow alternating-flow heat exchangers are disclosed in U.S. Pat. No. 4,271,110 to Minjolle, and U.S. Pat. No. 4,421,702 to Oda et al. U.S. Pat. No. 4,298,059 to Krauth et al. discloses a similar sawing method, wherein diamond cutting wheels are used to saw slots into an extruded body, after which time the ends of the slots are plugged to form an L-flow alternating-flow heat exchanger in which both flow paths through the heat exchanger follow an L-shaped path.
Thin-walled honeycomb substrates find extensive use as catalyst supports (such as in catalytic converters) where a honeycombed substrate is coated with a thin film having a high surface area, active metal oxide such as gamma alumina. Various techniques have been utilized to coat such substrates, including spraying and dipping, as disclosed in U.S. Pat. No. 3,565,830 to Keith et al. It is difficult to apply a uniform, thin coating even to conventional honeycomb substrates, as these substrates often are characterized by 200 or more cells per square inch (31 cells per square cm), with cells walls less than 0.02 inch (0.05 cm) thick. These difficulties are greatly magnified when the conventional honeycomb substrate is modified to be an alternating flow heat exchanger such as the L-flow, Z-flow, and U-flow devices described above, because of the non-straight secondary flow paths of such devices. The coating material has a tendency to collect at every turn or elbow in such non-straight flow paths. Further, in structures having non-straight flow paths, one cannot look from one end of the flow path to the other in such devices, making inspection of such devices (after washcoating) extremely difficult.
In many applications which utilize alternating flow devices, such as catalyst support applications, the objective is to maximize the overall surface area at which reactions can be promoted by a catalyst such as platinum. Consequently, uniform coating of the cell wall is important. If a coating is non-uniform, the efficiency of the catalyst device is reduced. The application of such uniform coatings can be very difficult in alternating flow structures, especially those having sharp directional turns (such as L-flow, Z-flow, or U-flow), as washcoat material can collect in these turns and lead to plugging of these paths. Such plugging greatly deteriorates the efficiency of the catalyst.
Thus, there is a need for a more efficient method to washcoat and/or catalyze alternating-flow heat exchangers having non-straight passageways. Preferably, such a method would also facilitate inspection of the washcoated or catalyzed product.